1. Field of the Invention
The present invention relates primarily to 4-stroke cycle diesel engines, and particularly, engines equipped with a DPF (diesel particulate filter), variable geometry turbocharger, and EGR (Exhaust Gas Recirculation) system which is composed such that a fraction of the exhaust gas joins together with the charged air in the cylinder of the engine by allowing the exhaust valve to lift a little in the suction stroke.
2. Description of the Related Art
There are 4-stroke cycle diesel engines and 4-stroke cycle gas engines that adopt an internal EGR system in which a fraction of the exhaust gas joins together with the charged air in the cylinders of the engine by allowing the exhaust valve to lift by a small lift in the suction stroke. This is called an exhaust valve sub-lift type internal EGR system.
The exhaust valve sub-lift type internal EGR system will be explained with reference to FIG. 5. In FIG. 5, the valve lift curve of the exhaust valve 7 and that of the inlet valve 5 versus crank angle are indicated by Es and In respectively. The exhaust valve lift Ex in the exhaust stroke is called here as the main exhaust valve lift. The exhaust valve 7 is lifted again by a small lift Hs in the suction stroke as shown by sub-lift Es of the exhaust valve 7. The exhaust valve sub-lift begins after closing of the main exhaust valve lift Ex. By lifting the exhaust valve by a small lift Hs in the suction stroke, a fraction of the exhaust gas in the exhaust passage 6 flows back into the cylinder. Thus, internal EGR is performed and generation Nox is reduced.
There are disclosed several types of internal EGR systems of internal combustion engines such as Japanese Laid-Open Patent Application No. 2006-226205 (patent literature 1), No. 7-133726 (patent literature 2), No. 10-252512 (patent literature 3), and No. 2000-204984 (patent literature 4), for example. The engine disclosed in patent literature 1 is provided with a DPF, an internal EGR system, and an external EGR system which includes an EGR conduit for introducing a fraction of exhaust gas in the exhaust pipe to the inlet passage and an EGR valve for controlling openings of the EGR conduit. The proportion of the internal EGR and external EGR is controlled to stabilize combustion and prevent the occurrence of rotational fluctuation of the engine when engine operation condition is returned from DPF recovering operation to low load operation.
In patent literature 2 is disclosed an inlet air control system in which an inlet control valve is provided in the inlet air passage. Negative pressure is formed in the inlet air passage by closing the inlet control valve and inlet valve in this order just before the end of the suction stroke. When the inlet valve is opened before the inlet control valve is opened just before the end of exhaust stroke, combustion gas in the cylinder flows into the inlet passage retained in negative pressure aided by the upward movement of the piston. The combustion gas flowed into the inlet passage flows into the cylinder together inlet air in the suction stroke as EGR gas. In this way, internal EGR is performed. By controlling negative pressure in the inlet passage in accordance with engine operating conditions, such as engine load and rotation speed, through controlling the inlet control valve or other means, the rate of EGR gas can be controlled.
According to the art disclosed in patent literature 3, the valve overlap period of the inlet and exhaust valves is increased to increase EGR amounts as engine loads decrease. By this heating effect of inlet air by EGR being increased, autoignition is enhanced, and a stable combustion range is widened when the engine is operated with a light load.
According to the art disclosed in patent literature 4, a variable valve timing mechanism and an exhaust throttle valve are provided, and when a large amount of internal EGR is required, inlet and exhaust valve opening timings are advanced together by means of a variable valve timing mechanism and further the amount of the exhaust throttle valve opening is decreased in order to allow a larger fraction of exhaust gas to be introduced into the engine cylinder.
In an engine equipped with the exhaust valve sub-lift type internal EGR system, the exhaust valve is lifted again in the inlet stroke by the small lift (Hs) as shown in FIG. 5 so that a fraction of exhaust gas in the exhaust passage flows back into the cylinder to be mixed with the charged air. When the flow resistance in the DPF located downstream of the turbocharger increases due to deposition of particulate matter, exhaust backpressure increases and inlet air flow to the engine decreases. As a result, excessive exhaust gas recirculation occurs, which enhances generation of particulate matter mainly composed of carbon particles and deposition of particulate matter in the DPF is enhanced. Further, when the internal EGR rate is excessively large, combustion temperature and exhaust temperature increase. When the exhaust temperature exceeds an upper limit, thermal load of the engine increases, and the increase in combustion temperature enhances generation of NOx and the effect of reducing NOx generation by EGR is decreased by the increased combustion temperature.
Particularly, with an engine provided with a particulate filter, when deposition of particulate matter in the particulate filter increases, exhaust backpressure increases due to increased flow resistance in the particulate filter. The increase in exhaust backpressure induces a decrease in air supply which induces an increase in internal EGR rate. By the increased internal EGR, combustion temperature and exhaust temperature increase and fear of inducing the problems as mentioned above is increased.
To solve the problems, it is conceivable to control by adjusting the exhaust valve sub-lift amount Es so that an internal EGR rate proper for a DPF difference pressure is attained, the internal EGR rate being a reference internal EGR rate determined in accordance with engine operation conditions, such as engine loads and engine rotation speed.
However, even when EGR rates are controlled as mentioned above, when DPF pressure difference is decreased by recovering operation of the DPF (burning operation of the particulate matter deposited in the DPF), internal EGR rates become lower than the reference internal EGR rate due to decreased DPF pressure difference. In this case, a problem arises that it is necessary to compensate for the decrease of the internal EGR rate.
According to the art of patent literature 1, a DPF, an internal EGR system, and an external EGR system are provided. Controlling of proportion of the internal EGR and external EGR to stabilize combustion and prevent the occurrence of rotational fluctuation of the engine when the engine operation condition is returned from DPF recovering operation to low load operation is disclosed in the literature, however a means to solve the problems mentioned above is not disclosed.
According to the art of patent literature 2, an inlet control valve is provided in the inlet passage and internal EGR amounts are controlled by controlling negative pressure generated in the inlet passage between the inlet control valve and inlet valve, and a means to solve the problems mentioned above is not disclosed.
In patent literature 3 is disclosed a compression ignition internal combustion engine, in which heating effect of inlet air by EGR is increased through increasing EGR amounts by increasing valve overlap period of the inlet and exhaust valves as engine loads decrease, and a means to solve the problems mentioned above is not disclosed.
According to the art disclosed in patent literature 4, inlet and exhaust valve opening timings are advanced together and further the amount of the exhaust throttle valve opening is decreased in order to allow a larger fraction of exhaust gas to be introduced into the engine cylinder when a large amount of internal EGR is required, and a means to solve the problems mentioned above is not disclosed.